Polymake.jl 0.3.1 released

Today Polymake.jl 0.3.1 has been released.

Diff since v0.3.0

Closed issues:

• shell execute errors upon showing the result (#223)
• No Sascha no release (#228)
• nvars for polynomials is a CxxWrap type (#235)

Merged pull requests:

• add some nicer logic/user facing errors to shell_execute (#227) (@kalmarek)
• Add bit operations for Integers (#229) (@saschatimme)
• add gcc version check to cmake (#230) (@benlorenz)
• Nemo dependency (#232) (@saschatimme)
• Add support for Array (#234) (@saschatimme)
• Add Array<Array> (#236) (@saschatimme)
• Fix: polynomial nvars (#237) (@saschatimme)